Generalized two-dimensional correlation NIR spectroscopy analysis of the structures on n-propanol and n-butanol

The hydrogen bond (H-bond) is one of the most important intra- and inter molecular attraction in chemistry, while the hydroxyl is one of the chemical groups that forms H-bond most easily. Therefore, a detailed understanding of the hydroxyl group is essential to a fundamental understanding of the H-bond. In this paper, the molecular structures especially the H-bond were investigated through the near infrared spectra (NIR) of n-propanol and n-butanol over a temperature range of 36–76 °C (or 36–79 °C) using two-dimensional correlation spectroscopy (2D-Correlation) techniques. The following results were drawned:1) the change of the hydroxyl vibrational frequency is caused by the breakage of H-bonds as the temperature increases, the vibrational frequencies of the H-bonds and free hydroxyl remain unchanged. However, the spectral peak positions shifted as the changed of H-bonds and free hydroxyl's composition. 2) when the temperature rose, the absorption of the free hydroxyl vibration in the first overtone region increased faster than H-bonded hydroxyl was reduced, which means that the frequency of H-bonded hydroxyl shifted to higher frequency before breaking. However, in the combination vibration region where both free hydroxyl and H-bonded hydroxyl overlapped, the situation is just the opposite; that is, the change of H-bonded hydroxyl is faster than in free hydroxyl. This difference may be related to the symmetry of hydroxyl's combination vibration. 3) With the increased temperature, the combination absorption will be partially converted to overtone vibration absorption, and the combination vibration absorption of n-propanol will be changed more easily into the first overtone absorption than is the case for n-butanol due to poor symmetry. 4) Principal component analysis (PCA) leads the authors to propose three possible states of hydroxyl bonding: free (non-H-bonding) hydroxyl, inter-hydroxyl group binding (through one or two H-bonds), and a weak force bonding (less bond energy than H-bonding) between the hydroxyl and alkyl groups on different molecules.